Geology and Soils in Deserts of the Southwestern United States - Design with the Desert: Conservation and Sustainable Development - page 51

FIGURE 2.9
A well-indurated K, or caliche , horizon (black arrows) forms the resistant ledge at the top of this stream wash in
the White Tank Mountains west of Phoenix, Arizona. (Courtesy of W.L. Stefanov.)
referred to as calcretes or caliche and can be so well indurated that heavy equipment or
dynamite is necessary to dig through them.
2.9 Landscape Organization of Soils
Soils vary across the landscape as a result in variation of the soil-forming factors from one
landscape position to another. In the hierarchical organization of soils, what is informally
known as a soil type is in fact a taxonomic group or “order.” In the metropolitan area of
Phoenix, the most common soil types are in the order Aridisols, which are soils of hot arid
regions, or Entisols, which are young soils that show little soil profile development (e.g.,
little development of soil horizons).
If soils were examined along a transect in the Sonoran Basin and Range from mountains
to the valley axis, several broad generalizations based on landscape position can be made
(Figure 2.10). In the mountainous regions, soils are typically shallow and dominated by
gravels or cobbles. The shallow nature of these soils is due in part to the slow rates of soil
formation in desert areas, a function primarily of the lack of water. Interestingly, the shal-
low nature of these soils is also driven by water erosion associated with monsoon rains
falling on surfaces of naturally low vegetative cover. These rains are responsible for the
removal and deposition of soil particles in lower landscape positions, especially alluvial
fans and pediments. Soils found on older alluvial surfaces (fans) are moderately coarse
textured, frequently with B horizons and clay dominated layers reflecting their great age.
It is the soils of these areas that are most likely to have well-developed K horizons or
caliche layers. In younger alluvial deposits, the clay layers thin out and soil horizons are
less developed. Soils of river floodplains are thick and range from coarse to fine-textured.
Little horizon development is apparent due to reworking of the soil materials by periodic
river flooding, leading to general classification of these soils as Entisols.